1. Field of the Invention
The present invention relates to calibration in image forming apparatuses such as color laser printers, color copiers, and color fax machines that mainly employ an electrophotographic process.
2. Description of the Related Art
In recent years, it has become mainstream for electrophotographic color image forming apparatuses to have a tandem-type configuration in which a photosensitive member is provided separately for each color in order to improve the printing speed. In tandem-type image forming apparatuses, color shift and density correction (i.e., calibration) is executed by, for example, forming a detection image, which is a pattern for detecting color shift and the density, on the intermediate transfer belt, and detecting reflected light from the detection image using an optical sensor. This calibration is executed at various timings, such as when a cartridge containing a photosensitive member is exchanged, after a predetermined number of sheets have been printed, after a predetermined time period has elapsed, or when there is a change in the hygrothermal environment.
When image formation is successively performed on multiple recording materials in an image forming apparatus, there are cases where the internal temperature of the apparatus rises, image formation conditions change, and the image density and tinting change. For this reason, Japanese Patent Laid-Open No. 2001-109219 discloses a configuration in which calibration is executed successively while successive printing is performed. Specifically, detection image formation and detection are performed in a region of the intermediate transfer belt that is between the trailing edge of one image that is to be transferred to a recording material and the leading edge of another image that is to be transferred to the next recording material (referred to hereinafter as the “non-transfer region”). The configuration disclosed in Japanese Patent Laid-Open No. 2001-109219 eliminates the need to interrupt image formation in order to execute calibration.
Also, Japanese Patent Laid-Open No. H03-134678 discloses an optical sensor for reading detection images. The optical sensor disclosed in Japanese Patent Laid-Open No. H03-134678 has an LED and a phototransistor that receives specular reflection light and diffuse reflection light when a detection image is irradiated with light from the LED. When the density is detected using the optical sensor disclosed in Japanese Patent Laid-Open No. H03-134678, the detection image that is to be read is formed by toner in an area that is approximately 10 mm×10 mm. Here, it is desirable that the detection image used for density correction is as small as possible. This is because the smaller the detection image is, the lower the toner consumption amount is, and the shorter the time required for calibration is. For this reason, Japanese Patent Laid-Open No. H07-020670 discloses a configuration in which a small detection image can be used due to using a CCD sensor. However, CCDs are expensive, and although the detection image can be small when a CCD is used, the cost of the image forming apparatus rises.
In an image forming apparatus that uses an intermediate transfer belt, a cleaning blade or the like for removing toner images from the intermediate transfer belt is arranged downstream of the secondary transfer roller, which is for transferring toner images to recording materials, in the direction in which the intermediate transfer belt rotates. For this reason, a detection image formed in the non-transfer region will attach to the secondary transfer roller, and this gives rise to an underside smearing phenomenon in which toner in the detection image attaches to the underside of the subsequently passing recording material. This underside smearing can be prevented by causing the transfer bias output by the secondary transfer roller to be switched to the opposite polarity when the detection image passes the secondary transfer roller. However, even when control for switching the transfer bias to the opposite polarity is performed, if the density of the detection image is high, it remains that detection image will attach to the secondary transfer roller.